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324 ranged in age from approximately 35 to 45 years. The root-stem transition zone was visible prior to the ini- tiation of the study. To evaluate how force moves into the root system in relation to the direction of loading, 3 roots per test tree were examined. The root orientations were labeled as follows: leeward roots in the direction of the pull- ing force; windward roots in the opposite direction of the pulling force; and the tangential roots located per- pendicular to the direction of the pulling force. Tan- gential roots could be found on either side of the pulling direction, as depicted by root morphology or physical limitation. An Air Knife (Allison Park, PA, USA) was used to excavate the root zone adjacent to the base of each tree. First order roots were exposed for 1 m distally from the base of each tree trunk to a depth of approx- imately 15 cm below grade. Soil excavation was required to expose the root tissue surface for applying the paint for DIC measurements. Fine woody roots were removed with small hand pruners to provide a direct line of sight to the exposed structural roots. DIC imagery requires a moderately even surface, as thick ridges or deep furrows create a shadowing effect and obstruct the view from one or both cam- eras. Some of the larger roots had deeply furrowed areas on the RSTZ, and the thick bark ridges (primary phloem) were reduced by means of a draw knife. Care was taken to avoid damaging the secondary phloem. After initial surface preparation, a white back- ground was applied using a flat, nonglossy paint, fol- lowed by a stochastic black speckling pattern. After the paint was allowed to dry, reference marks were placed with a permanent marker at 2.5-cm increments along the main axis of the root, starting near the top edge of the painted RSTZ and continuing distally along the main root (Figure 4). As the roots were not perfectly in line with or per- pendicular to the direction of pull, root divergence was a measurement (in degrees) of how a root aligned with the pulling direction. Specifically, root diver- gence was calculated as the number of degrees (using a compass) that the general direction of root growth differed from the line of pulling (leeward and wind- ward). In the case of tangential roots, root divergence was calculated as the number of degrees from 90° perpendicular to the direction of pull. Soil moisture was measured at three locations, one each on both sides of the given root and one on the distal end. ©2020 International Society of Arboriculture Beezley et al: Strain in Roots in Urban Trees Figure 4. RSTZ and root prepared for sampling with painted surface and 2.5-cm reference lines on root. Readings were taken during the same day of sampling for each tree using a Campbell Scientific HydroSense soil moisture probe (Campbell Scientific, Logan, UT, USA) and then averaged. The static pull line was secured in the tree using a webbed sling that was placed below any codominant branching or lateral limbs that would affect the pull- ing process (Beezley et al. 2016). A 44-kN capacity load cell (Interface, Scottsdale, AZ, USA) recorded the applied load. The height of the pulling line attach- ment and the stem diameter at the point of attachment were measured and recorded. A 44-kN capacity elec- tric winch (Warn Industries, Inc., Clackamas, Oregon, USA) was attached to the base of the anchor tree using a webbed sling. The tilt of the tree stem was measured using a 22.9-cm-long digital level (Husky, Atlanta, GA, USA) that was mounted on 2 aluminum nails. The center of the level was 42.5 cm above the ground, 180° oppo- site of the pulling direction. A digital video recorder was positioned to record the measurement of the dig- ital level and to capture audio during each pulling trial for use in post processing. The trees were pulled to a stem deflection to 0.1° from natural lean. Figure 5 displays the static loading setup used for this research. Bending moment was calculated as MBend = [Cos θ × F] × HeightA, where θ is the rope angle, F is the applied
September 2020
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